1. Field of the Invention
The present invention relates to valve train control systems for reciprocating piston internal combustion engines and more specifically to valve train control systems for variably altering the duration and/or timing characteristics of the valves during crankshaft/camshaft rotation.
2. Background Art
FIG. 1 depicts a conventional cam lobe and valve interface, generally designated 30, for a single valve (intake or exhaust) in an overhead cam system. As readily understood by one of ordinary skill in the art, a camshaft 32 coupled to the crankshaft of a combustion engine is machined or cast to include one or more cam lobes 34 that include an peripheral working surface 36 defined by the cam lobe profile 38. The cam lobe profile includes a broad shallow region or base circle 40 for a zero lift position when the valve is seated and the combustion chamber closed, a more narrow opposing region or nose 42 for a maximum valve lift position opening a pathway to the combustion chamber, and a pair of opposing relatively large radius of curvature transition regions or flanks 44a, 44b in between.
With continued reference to the conventional configuration in FIG. 1, the cam lobe 34 interacts with a valve, generally designated 46, that includes a cam follower 48 and a spring retainer 56 cooperating to form a cam follower and spring retainer assembly, an elongated valve stem 50, and a valve head 52 for sealing off the combustion chamber. At the top end of the cam follower 48, a planar upper cam following surface 54 interacts with and follows the rotating peripheral working surface 36 of the cam lobe 34 as the cam lobe rotates with the camshaft 32 to open and close the valve 46 in operation allowing air to enter the combustion chamber if an intake valve or exhaust gases to exit if an exhaust valve. A biasing element such as a spring (not shown) is maintained at least partially within the spring retainer 56 and typically biases the valve head 52 toward a closed position against the valve seat (not shown in FIG. 1). The force of the cam lobe working surface 36 against the valve follower surface 54 must overcome this spring force to drive the valve into an open position. In operation, the rotational motion of the camshaft 32 and cam lobe 34 is translated into a reciprocal linear valve motion. It will be appreciated that the foregoing would be readily understood by one of ordinary skill in the art but has been merely provided as a matter of general background. It will also be appreciated that the camshaft may incorporate one or more cam lobes that interact with a corresponding set of intake and exhaust valves and that one or more intermediate components (not shown) such as pushrods, finger followers, and/or rocker arms may be interposed in the operational path between the cam lobe and the follower depending on the valve train configuration.
In the normal four-stroke (Otto Cycle) internal combustion engine as it now exists, each cam lobe on one or more camshafts controls the opening and closing of individual intake and exhaust valves. A camshaft is driven at half the crankshaft rotational speed. Operation of a four-stroke internal combustion engine consists of four separate one-half rotation cycles of the crankshaft, each one moving the piston from either top to bottom of its stroke or vice versa as follows: 1) the intake stroke is when a piston moves from top to bottom in its cylinder, sucking in the air/fuel mixture with the intake valve open; 2) the compression stroke is when the same piston moves from the bottom to top in its cylinder, compressing the air/fuel mixture with both intake and exhaust valves closed; 3) the power stroke is when the same piston moves from top to bottom in its cylinder after the spark plug ignites the air/fuel mixture; and 4) the exhaust stroke is when the same piston moves from bottom to top, pushing the burned air/fuel mixture out of the cylinder with the exhaust valve open.
If the cam lobe timing and duration is such that the intake and exhaust valves are only open during the extent of their respective stroke, the engine would be restricted to low crankshaft rotations per minute (RPM) operation and low total engine power output. This RPM operating range would offer extremely low idle speeds (conserving fuel) and high torque values at low-speed. Due to acceleration and deceleration limits of such a valve train, the area under a graphical plot of valve lift versus crankshaft rotation for “stroke extent” timing/duration would be very low, however.
At high RPM, the breathing efficiency would be too low for any significant level of power. To raise the breathing efficiency to a level required for useable total engine power necessitates taking advantage of the momentum of the air/fuel mixture as it starts and stops flowing past the valves near the closed position. Conventional testing and experience has taught that the higher the RPM desired for power, the longer the duration of each cam lobe needs to be. This extension of duration timing at the opening and closing of the valves is not uniform but it greatly increases the area under a plot of the lift/duration curve.
Since the end of the exhaust cycle corresponds with the beginning of the intake cycle (at the top of piston travel), any extension of the duration for cam lobes results in valve “overlap”, where both valves are open at the same time. During this “overlap” period, the exhaust valve is finishing its closing phase while the intake valve is starting its opening phase.
Valve openings and closings near the bottom of piston travel have different parameters from those at the top of piston travel. Extending the closing of the intake valve timing into the beginning of the compression stroke takes into account the momentum of the intake charge near the bottom of piston travel. A considerable amount of intake valve opening is desirable at the bottom of the piston stroke. Extending the opening of the exhaust valve early into the end of the power stroke provides more time for the high-pressure exhaust gases to exit the cylinder. An additional factor allowing considerable timing extension of these two events is the crankshaft rotational position/connecting rod angle relationship. The piston does not exhibit a lot of motion in the cylinder near bottom of travel compared to top of travel. Valve duration timing of these two events is considerably more extended than the overlap timing.
If the cam lobe timing and duration allowed sufficient overlap of valve opening and closing, along with late intake valve closing and early exhaust valve opening, breathing efficiency at high RPM would be greatly increased. Although at very low RPM, efficiency would be impaired, fuel economy reduced, and pollution increased.
Fixed duration and timing of the valve cam lobes yields good results only in a fairly narrow range of RPM, and is a significant compromise in the other operational RPM bands. Current engine technology has added the capability to vary the cam lobe timing phase, in some cases intake and exhaust individually, but this amounts to little more than fine tuning compared to what is desirable. A typical lift profile curve for a conventional engine without any timing variations is shown in FIG. 14 while that of a valve train incorporating phase (timing) shifting technology is shown in FIG. 15. A currently unattained ideal system is shown in FIG. 16 and a lift profile curve for cam switching technology such as for Honda's VTEC engine system is shown in FIG. 18. Each of these are discussed below in more detail.
No mechanism exists in practice today that permits continuous variable control of the duration of valve opening during engine operation. Such valve duration variation is necessary for optimizing engine function with varying conditions to meet current and anticipated federal regulations for fuel consumption and exhaust pollution. The principal change in engine operating conditions, for which variable valve open duration is advantageous, is change in RPM. Other engine operating parameters such as throttle position, manifold vacuum, air temperature and pressure have a smaller effect on engine operating conditions and may be employed as signals for valve open duration adjustment. For diesel engines, compression ratio and cylinder pressure are significant factors that would add to the RPM and fuel feed rate (equivalent throttle position) determination for the valve duration and timing.
Given the drawbacks of conventional valve train technology, there exists a need for an engine system incorporating an improved variable valve train control device for altering the valve characteristics during crankshaft/camshaft rotation to serve a wider range of engine speeds using a single cam lobe profile set.